I'm mainly noticing two bugs.
The first has been widely reported--spotlightnethelper is "crash-y." When it's unresponsive, Spotlight functionality like "Now Playing in Theaters" does not work. That functionality is not restored by force-quitting the spotlightnethelper. Short of restarting the computer, disabling and then reenabling Location Services does restore that functionality, at least until spotlightnethelper crashes again.
The second bug I've not seen reported. It occurs with applications that have helper apps in an application package (Contents/Library/LoginItems) in order to launch on login using the Service Management Framework. Many of these applications allow you to toggle launch on login on and off right in the app's preferences. The problem is, toggling start on login from "on" to "off" does not seem to "stick" once I logout or restart. Upon restart/logging in, the apps will still launch, and a look at the preferences shows start on login again checked. Examples include the Mac App Store apps Living Earth Desktop, Command-C, and Dialogue. Can anyone verify whether you too are experiencing this? I just want to make sure that it's a replicable "bug" and not something messed up with my installation.
Other than that, I find Yosemite to be pretty stable. With 2.5 months to go until finalization, I think that this is going to be one of the most rock-solid 10.X.0 releases in a long while.